1. Field of the Invention
The present invention relates to a bonding coordinate teaching method and system used in a wire bonding apparatus.
2. Prior Art
A workpiece 3 shown in FIG. 5 includes a semiconductor chip 2 bonded to a lead frame 1 via wires 4. The wires 4 are connected between the pads P.sub.1, P.sub.2 . . . of the semiconductor chip 2 and the leads L.sub.1, L.sub.2 . . . of the lead frame 1. The bonding between the pads and the leads via the wires is accomplished by a wire bonding apparatus which is, for example, shown in FIG. 6.
Generally, when the wire bonding is executed, any positional discrepancy between the semiconductor chip and the lead frame is first corrected. The correction is made in the following manner: First, a shift or deviation between at least two points on a semiconductor chip and at least two points on a lead frame from a datum position is detected, and then the bonding coordinates stored beforehand in the memory of a bonding apparatus are corrected by an arithmetic unit based upon the detected values of the shift.
More specifically, when the shift or discrepancy detection is performed by a camera, as seen in FIG. 6, an X-axis motor 12 and a Y-axis motor 13 are driven so that the central (or optical) axis 11a of the camera 11 is positioned directly above the point to be detected. After the bonding coordinates have been corrected as described above, a capillary 15 is moved in the X, Y and Z directions so as to be above the point to be detected, and then the wire 4 which is passed through the capillary 15 is wire-bonded.
In this case, the central axis 11a of the camera 11 is provided at an offset position relative to the central axis 15a of the capillary 15 by a distance of W so that the XY table 16 is moved by an offset amount of W by the X-axis motor 12 and Y-axis motor 13 so that the capillary 15 is positioned above the first bonding point.
Afterward, the wire 4 is wire-bonded at the corrected bonding coordinate by moving the XY table in the X and Y directions by the X-axis motor 12 and Y-axis motor 13, and by further moving the capillary 15 in the Z direction by the vertical movement (or pivoting) of the capillary arm 17 that is effected by the Z-axis motor 14.
In the above, the offset amount W is shown by Equation 1 below, wherein Xw is the X-axis component, and Yw is the Y-axis component:
[Equation 1] EQU W=(Xw.sup.2 +Yw.sup.2).sup.1/2
In the wire bonding method described above, it is necessary to input the respective bonding coordinates into the memory of the bonding apparatus before the wire bonding operation is executed. The input of such is also necessary when the type of workpiece to be processed is changed. The input into the bonding apparatus as described above is generally accomplished by a teaching method.
The existing teaching method used for the respective bonding coordinates of the pads P.sub.1, P.sub.2 . . . is performed by the following steps:
(1) a manual input means such as a ten-key, chessman, etc., which moves the X-axis motor 12 and Y-axis motor 13, is manually operated by an operator so that the coordinates (x.sub.1, y.sub.1) of the first pad P.sub.1 are inputted. Such coordinates are shown on a television monitor, PA1 (2) then while viewing the monitor screen, the operator operates the manual input means so that the position that is thought to be the center of the pad P.sub.1 is aligned with cross-hairs shown in the center of the monitor screen, and this coordinate position is stored in the memory of the bonding apparatus, PA1 (3) the operation described above is performed for all of the pads P.sub.1, P.sub.2 . . . . PA1 (1) for the first and second pads: PA1 (2) for the third pad: PA1 (3) for the all subsequent pads (fourth pads and on), the steps above are repeated. PA1 (a) the image control unit (CPU) of the image processing section processes the images obtained by the camera and then calculates the amount of shift of the centers of the images of the target pads; PA1 (b) the drive control unit of the main drive section functions such that for the first and second pads, the camera is moved to positions directly above the pads by a manual input means and the bonding coordinates of the first and second pads are inputted, then the inputted bonding coordinates are corrected based upon the amount of shift of the centers of the images calculated by the image control unit, thus forming new bonding coordinates for the first and second pads; and for the third and all the subsequent pads, the difference between the new bonding coordinates for the preceding two pads (that is, the coordinates of the first and second pads if the third pad is a target pad) are calculated, the camera is moved a distance that corresponds to the thus calculated difference of preceding coordinates, and the coordinates to which the camera has been moved are corrected based upon the amount of shift of the center of the image of the target pad calculated by the image control unit, thus forming next new bonding coordinates (for the third and the following pads), and PA1 (c) the bonding coordinate memory stores new bonding coordinates one after another for the subsequent pads.
In the above described prior art method, teaching of the bonding coordinates is accomplished by the operator who operates the manual input means in accordance with a predetermined order, and the alignment operation is performed a number of times that is equal to the number of pairs of coordinates to be stored in the memory.
Due to such complex operations, several problems arise. The operation requires a considerable amount of time, and positioning errors would occur as a result of mistakes made by operators, particularly by individual differences between the operators. Furthermore, in such a manual input, the resolution of the object image solely depends upon the resolution of the monitor; and as a result, positions finer than what appears on the monitor screen is not able to be judged. Thus, the positioning precision ought to depend upon the pixel or picture element units of the monitor.